This invention relates to chromatographic apparatus used to identify the chemical constituents of material samples. More particularly, this invention relates to control circuitry used in connection with such chromatographic devices.
Chromatographic devices are known which are used in the analysis of chemical constituents in sample materials. Such devices typically include an injector port for enabling the injection of a sample to be analyzed into the inlet portion of the chromatographic device, a column providing a transport path for the injected sample, a detector positioned at the end of the column for generating variable amplitude samples representative of the quantity of analyte (sample material) passing through the detector, and electromechanical circuitry for recording the detector signal and for controlling the operation of the various electrical and mechanical components of the device. In gas chromatographs, a carrier gas inlet is provided at the injector port for enabling the introduction of a carrier gas, which serves as the transport vehicle through the column for samples introduced into the injector port. In capillary column chromatographs, an outlet, usually termed a purge or split exit line, is typically included to provide an outlet path to the atmosphere or ambient for the carrier gas and also maintain the fluid pressure between the inlet and the outlet at some predetermined value or within some predetermined range.
One of the major information components generated during chromatographic analysis of a sample is the length of time between the start of the sample injection to the appearance of a particular chromatographic peak in the detector signal: by measuring the precise time interval, the retention time of a given chromatographic peak in the detector signal can be compared with known retention times of individual chemical constituents in order to identify the particular constituent associated to a given chromatographic peak in the detector signal. Consequently, a necessary step in the chromatographic analysis process is the generation of a starting reference time signal. In the past, the starting time reference signal has been provided in one of two ways: manually or automatically. According to a first manual technique, a manual switch is provided for a timer or clock associated with the chromatographic apparatus and the operator manually actuates the switch at the same time the sample is manually injected into the injector port. In a second manual technique, a mechanical switch is provided at the injector port in such a location that the switch is mechanically triggered upon insertion of a syringe needle into the injector port.
Both of the above manual techniques for providing an initial timing signal suffer from the disadvantage of providing a relatively imprecise definition of the instant of sample injection. In the first manual technique, the accuracy of the manual switch closure depends upon the ability of the operator to inject the sample and operate the switch simultaneously. The second manual technique employing the mechanical switch measures the instant of insertion of the syringe needle into the injector port, which may or may not coincide with the instant at which the sample is released from the syringe. Perhaps most importantly, however, neither technique provides a precise indication of the moment at which the sample enters the injector port. As a consequence of the above disadvantages, an element of uncertainty is introduced into the chromatographic analysis process, which is highly undesirable.
The automatic technique for generating the starting time reference signal employs a complicated and expensive pneumatic or electromechanical apparatus, usually sold as auxiliary equipment, which generates a signal when a sample containing syringe is actuated to mechanically inject a sample into the injector port. Since this apparatus is used for automatic loading of samples into the syringe and automatic sample injection into the injection port, it is not adapted to be conveniently employed to generate a starting time reference signal for single runs or for runs of only a few samples. In addition, since the actuation of the syringe is the event being monitored, the starting time reference signal generated by this technique is not an exact measure of the time at which the sample actually enters the injector port.